1. Field of Invention
The present invention relates to an optical system and an exposure apparatus provided with the optical system, and more particularly relates to a projection optical system suitable for an exposure apparatus used to fabricate microdevices such as semiconductor devices using a photolithography process.
2. Description of Related Art
In recent years, considerable progress has been made in making linewidths finer when fabricating semiconductor devices or liquid crystal display devices, and the need has arisen for projection optical systems of greater resolution in exposure apparatus that form the patterns. In order to satisfy this need for higher resolution, it is necessary to shorten the wavelength of the exposing light and also enlarge the NA (numerical aperture) of the projection optical system. However, when the wavelength of the exposing radiation (light) becomes short, there are limitations on the type of optical glass that can be used due to light absorption.
For example, when light in the vacuum ultraviolet region having a wavelength of 200 nm or less, particularly F2 laser light (157 nm wavelength), is used as exposure light, it is necessary to make abundant use of fluoride crystals such as calcium fluoride (fluorite: CaF2) or barium fluoride (BaF2) as the radiation transmissive optical material comprising the projection optical system. In actuality, designs exist specifically for making a projection optical system out of fluorite alone in an exposure apparatus that uses F2 laser light as the exposing light. Fluorite is a cubic (isometric) system, is optically isotropic and was thought to have effectively no birefringence. In addition, in experiments in the conventional visible light wavelength range, only a small birefringence (a random phenomenon caused by internal stress) was observed in fluorite.
However, at the 2nd International Symposium on 157 nm Lithography held on May 15, 2001, John H. Burnett, et. al, of the U.S. National Institute of Standards and Technology announced that they had confirmed both experimentally and theoretically the existence of intrinsic birefringence in fluorite.
According to this announcement, birefringence in fluorite is essentially zero in the direction of the crystal axis [111] and in the direction of its optically equivalent crystal axes [-111], [1-11] and [11-1], and in the direction of the crystal axis [100] and in the direction of its optically equivalent crystal axes [010] and [001], but has a substantially nonzero value in other directions. In particular, in the six directions of the crystal axes [110], [-110], [101], [-101], [011] and [01-1], fluorite has a birefringence of up to 6.5 nm/cm for the 157 nm wavelength and up to 3.6 nm/cm for the 193 nm wavelength. These birefringence values are effectively larger than the 1 nm/cm permissible value for random birefringence. Moreover, there is a possibility that the effects of birefringence could add up from the nonrandom portion passing through multiple lenses.
In the prior art, the birefringence of fluorite is not taken into consideration in the design of projection optical systems, and in general the crystal axis [111] is made to coincide with the optical axis from the standpoint of ease in processing. In this case, the NA (numerical aperture) is relatively large in the projection optical system, and consequently light beams with a certain degree of inclination from the crystal axis [111] pass through the lenses. As a result, it is possible that the imaging performance could deteriorate due to the effects of birefringence.
However, in the aforementioned announcement by Burnett et al., a method was disclosed for correcting the effects of birefringence by having the optical axes and crystal axes [111] of a pair of fluorite lenses coincide, and rotating the pair of fluorite lenses 60° relative to each other about the optical axis. However, with this method, although it is possible to mitigate the effects of birefringence to a certain extent, as noted above this does not positively correct for the effects of birefringence in the direction opposite to this. As a result, the correction efficacy is inadequate.